Instead of using all data as we have done so far using
the SELECT keyword, you can present a condition that the database would follow to isolate specific records.
One of
the keywords you can use to formulate conditions is WHERE. Its basic syntax is:

SELECT What FROM WhatObject WHERE Expression;

The expressions used in conditions are built using
algebraic, logical, and string operators. The Expression factor is
called a criterion. Although a group of expressions, making it plural is
called criteria, the word criteria is sometimes used for a singular
expression also.

The expression is written using the formula:

ColumnName=Value

The ColumnName factor must be an existing
column of the table. It is followed by the assignment operator. The Value
factor is the value that would set the condition. If the value is a word
or a group of words (also called a string), you must include it in
single-quotes. Here is an example:

In Boolean algebra, something is considered TRUE
when it holds a value. The value is also considered as 1 or Yes. By
contrast, if something doesn't hold a value, it is considered
non-existent and non-worthy of consideration. Such a thing has a value
of FALSE, 0, or No. To retrieve such a value, you can just find
out if the value of a field exists or not.

The comparison for a True or False value can be performed on
a Boolean column. If a record has a value of 1, the
table considers that such a field is True. If the field has a 0 value,
then it holds a FALSE value.

The NULL Constant

While the True and False constants are used to find
out whether a Boolean expression has a positive or a negative value, the database provides another
constant used to find out whether a field is empty. This can be done
using the NULL constant.

When a field holds a value, the value would be
considered using the comparison operators we will learn shortly. If the
field is empty, it holds a value of NULL.

Filtering Records With Boolean Operators

Equality =

The equality operator is used to compare two values
for similarity. The syntax of this operation is:

Value1 = Value2

If Value1 and Value2 hold the same value, the
comparison produces a TRUE result. If they hold different values,
the comparison renders a FALSE value. Here is an example:

To find out if two fields hold different values, you
can use the inequality operator which is represented by <>. Its
syntax is:

Value1 <> Value2

This comparison is performed between Value1 and
Value2. If they hold different values, then the comparison produces a TRUE
value. If they hold the same value, the comparison produces FALSE.
This shows that the equality (=) and the inequality (<>) operators
are opposite each other. Here is an example:

If Value1 holds a value that is lower than that of
Value2, the comparison produces TRUE. If Value1 holds a value
that is greater than or similar to that of Value2, the comparison
renders FALSE.

Less Than or Equal <=

When comparing two values, you may want to know
whether two fields hold the same value or if one is lower than the
other. This comparison can be performed with the "Less Than Or
Equal To" operator. It is represented by <= and its formula is:

Value1 <= Value2

If both operands (Value1 and Value2) hold the same
value, then the comparison produces a TRUE result. If Value1
holds a value that is lower than that of Value2, the comparison still
produces a TRUE result. By contrast, if the value of Value1 is
higher than that of Value2, the comparison renders a FALSE
result. Notice that the > and the <= operators are opposite each
other.

Greater Than >

The > operator is used to find out whether one
value is "Greater Than" another. Its syntax is:

Value1 > Value2

The operation is performed on the values of Value1
and Value2. If Value1 holds a value greater than that of Value2, then
the comparison produces TRUE. Otherwise, the comparison produces FALSE.
That is, if the value of Value2 is greater than or equal to that of
Value1, then the comparison produces FALSE.

Greater Than or Equal >=

If you have two values and want to find out whether
they hold similar values or the first is greater than the second, you
can use the >= operator whose syntax is:

Value1 >= Value2

If both Value1 and Value2 hold the same value, then
the comparison renders a TRUE result. Similarly, if the left
operand, Value1, holds a value greater than that of the right operand,
Value2, the comparison still produces TRUE. If the value of
Value1 is less than the value of Value2, the comparison produces a FALSE
result. Therefore, < and >= are opposite.

The NOT Operator

To deny the presence, the availability, or the
existence of a value, you can use the NOT operator. This operator
is primarily used to reverse a Boolean value. For example, we have learned
that FALSE is the opposite of TRUE. In the same way, TRUE
is the opposite of FALSE. If you want to compare a value as not
being TRUE, the NOT TRUE would produce the same result as
the FALSE value. For the same reason, the expression NOT FALSE is
the same as TRUE.